Standard electro-acoustic transducers, such as microphones and speakers convert mechanical vibrations of diaphragms into electric signals. Microphones sense vibrations of a diaphragm in response to sound waves in the form of, for example, variations in electromagnetic property, electrostatic capacity, or opto-electric property to convert the variation into electrical signals. Common speakers electromagnetically convert audio signals into vibrations of a diaphragm and emit sound waves. The diaphragms of these electro-acoustic transducers are used to convert air vibrations into electrical signals and to convert electrical signals into air vibrations.
Control schemes for electro-acoustic transducers based on machine vibration systems provided with diaphragms includes a mass control, a resistance control, and an elastic control. The resonant frequencies of a diaphragm are designed to be located near the lower limit, at the center, and near the upper limit of the main frequency band. Conventional electro-acoustic transducers with diaphragms, which have been commonly used, in particular microphones have a limited frequency response due to the existence of the diaphragms in any control scheme. Even if the mass of the diaphragm is reduced to the utmost, the remaining mass causes inertia force leading to limited sound collection in some frequency.
Diaphragm-free electro-acoustic transducers have been investigated to solve such a limitation. As an example production of such investigations, Japanese Unexamined Patent Application Publication No. 55-140400 discloses a method for detecting the velocity of particles generated by electric discharge and electro-acoustically converting the velocity. In this disclosure, a counter electrode interspatially surrounds a needle electric discharge electrode. The counter electrode is composed of a sphere conductive material having holes for transmitting sound waves. The electric discharge electrode extends to the interior of the spherical counter electrode and reaches the substantial center of the sphere. RF voltage signals are applied to the electric discharge electrode from an RF voltage generating circuit. This RF voltage signals are modulated with low frequency signals to be converted into sound waves in the RF voltage generating circuit. Corona discharge in response to the RF voltage signals occurs between the electric discharge electrode and the counter electrode to emit the low frequency signals, i.e., sound waves.
The invention described in Japanese Unexamined Patent Application Publication No. 55-140400 relates to an ionic speaker and is not assumed to be used as a microphone. The present inventor had proposed a microphone which can convert sound waves into electrical signals by electric discharge (see Japanese Unexamined Patent Application Publication No. 2010-183330).
The microphone described in Japanese Unexamined Patent Application Publication No. 2010-183330 includes needle electrode, a counter electrode facing the needle electrode, an electric discharger provided between the needle electrode and the counter electrode, an RF oscillation circuit including the electric discharger and generating RF electric discharge in the electric discharger, a sound wave guide introducing sound waves into the electric discharger, and a modulation signal output terminal extracting signals modulated in response to sound waves oscillated in the RF oscillation circuit and introduced into the electric discharger. The RF oscillation circuit RF-oscillates at the electric discharger as a feedback path between the needle electrode and the counter electrode. The electric discharge unit discharges RF waves. The equivalent impedance of the electric discharger then varies in response to sound waves and is frequency-modulated. Sound waves, i.e., audio signals is obtained by demodulating the frequency-modulated signals.
Examples of electro-acoustic transducers with an RF electric discharge scheme as described in Japanese Unexamined Patent Application Publications Nos. 55-140400 and 2010-183330 include ionic speakers (ionic tweeters). The technique described in Japanese Unexamined Patent Application Publication No. 2010-183330 can be applied to ionic microphones. According to Japanese Unexamined Patent Application Publication No. 2010-183330, an RF voltage is applied between the needle electrode and the counter electrode facing each other to generate from the tip of the needle electrode plasma toward the counter electrode. The plasma is generated like a flame from the needle electrode toward the counter electrode and may therefore be called an electric discharge flame. The needle electrode in contact with the plasma has a high temperature.
It was found that a cylindrical electrode facing the needle electrode, instead of a plate electrode, can enhance the sensitivity of the electro-acoustic transducer. The sensitivity of the electro-acoustic transducer can also be enhanced by increasing electric discharge power. Excess electric discharge power however causes a transition to spark discharge between the electrodes to cause a substantial short circuit. In consequence, these traditional techniques are not suitable for electro-acoustic transducers. The structure of the cylindrical electrode facing and surrounding the needle electrode, as described above readily generates spark discharge. The reason will be explained below.
FIG. 8 illustrates a typical conventional electro-acoustic transducer with RF electric discharge. In FIG. 8, the counter electrode 4 facing the needle electrode 3 has a cylindrical shape. The tip of the needle electrode 3 is adjacent to and continuous with the counter electrode 4 in the direction of the central axis of the needle electrode 3 and the counter electrode 4. The needle electrode 3 and the counter electrode 4 have the common central axis. As seen in the direction of the central axis of the needle electrode 3 and the counter electrode 4, the counter electrode 4 surrounds the outer circumference of the needle electrode 3 with a predetermined gap. The base of the needle electrode 3 is covered with an insulating cylinder 5. The insulating cylinder 5 is further fit into an insulating cylinder 6. The insulating cylinder 6 penetrates across the thickness of a disk base 1 and is fixed with the base 1.
The outer circumference of the base 1 is fit into the inner circumference at one end of the cylindrical case 2. The case 2 extends from the outer surface of the base 1 along the needle electrode 3. The needle electrode 3 extends through the space defined by the case 2 substantially on the central axis of the case 2. The opposite end of the case 2 to the fixed base 1 is open. This opening end has an inner circumference fit into the outer circumference of the counter electrode 4. The counter electrode 4 is thereby fixed on the case 2.
An RF voltage is applied to the needle electrode 3 from a driver 7 including, for example, an RF oscillation circuit. The needle electrode 3 and the counter electrode 4 define an electric discharger. An RF voltage is applied to the electric discharger to discharge RF waves in the electric discharger. The electric discharge is called torch discharge. FIG. 8 illustrates an electric discharge flame 8, i.e., plasma, generated by electric discharge in the electric discharger.
An electro-acoustic transducer is usually placed such that sound waves enter or emit in a lateral direction. In other words, an electro-acoustic transducer is used on an appropriately upward or downward slant from a horizontal state if necessary. FIG. 8 illustrates a normal state of the electro-acoustic transducer. The electro-acoustic transducer is positioned such that sound waves enter from the left or emit to the left. The plasma 8 is gas containing charged particles generated by ionization and has a high temperature. If the electro-acoustic transducer is positioned laterally as illustrated in FIG. 8, the plasma 8, which is hot gas, extends from the tip of the needle electrode 3 along the central axis of the counter electrode 4. In use, the tip of the plasma 8 curves upward due to the temperature rise.
The counter electrode 4 has a cylindrical shape. When the tip of the plasma 8 curves upward as illustrated in FIG. 8, the plasma 8 may reach the counter electrode 4. The plasma 8 reaching the counter electrode 4 leads to the transition from plasma electric discharge to spark discharge. The spark discharge is equivalent to a short circuit between the needle electrode 3 and the counter electrode 4 and disables the electro-acoustic conversion. This technical problem was found by a cylindrical counter electrode 4 provided to enhance the sensitivity of the electro-acoustic transducer. An increase in an RF voltage applied to the needle electrode 3 can enhance the sensitivity of the electro-acoustic transducer. Such a configuration however causes an increase in the size and the curve of the tip of plasma 8 and readily leads to a short circuit between the needle electrode 3 and the counter electrode 4.
In order to prevent the influence of the above technical problem as illustrated in FIG. 9, the electro-acoustic transducer may be positioned upright along a vertical central axis. The needle electrode 3 is positioned upright in the vertical direction, and the plasma 8 vertically extends from the tip of the needle electrode 3 without distortion. The shape of the plasma 8 does not change at this attitude even if the temperature rises. The counter electrode 4 surrounds the plasma 8 with a predetermined gap from the circumference of the plasma 8.
According to the electro-acoustic transducer as illustrated in FIG. 9, no short circuit occurs between the needle electrode 3 and the cylindrical counter electrode 4 even for a higher RF voltage applied to the needle electrode 3 and a larger plasma 8. Unfortunately, electro-acoustic transducers, i.e., microphones and speakers are rarely used upright as illustrated in FIG. 9. In use as a microphone in particular, sound should be applied to the electro-acoustic transducer from right above toward right below, and this leads to the influence of a high temperature of the plasma 8. It has therefore been required to provide a structure not causing a short circuit between the needle electrode 3 and the counter electrode 4 due to the plasma 8 even in the lateral position in use as illustrated in FIG. 8.